This project will provide the capability to maintain pedigreed breeding colonies of feral mice from various geographical areas of the world. The mice will be used to test the effect of various biological (hormones, mouse mammary tumor virus (MMTV), and recombinant DNA expression vectors) and chemical carcinogens on the incidence of mammary tumors and to provide a source of tumor tissue to define the organization and to test for the expression of tumor-associated genes and retroviral genes.